The present invention generally relates to an electrographic process and more particularly, to a method of detecting and controlling toner concentration for an electrographic copying apparatus which is provided with a developing device arranged to develop an electrostatic latent image through employment of a developing material composed of a mixture of toner and carrier.
Although various kinds of toner concentration control methods of the above described type have been proposed, they have such disadvantages that complicated arrangements are required therefor or control of toner concentration is not accurately effected at all times. Among the methods as referred to above, as a practice suitable for a toner image transfer type electrophotographic copying apparatus, there has been known one method which includes such steps as, where a reference image is provided at a leading edge or trailing edge of an original document platform, the reference image is projected for exposure onto a corresponding leading edge or trailing edge of an image area of a photosensitive member (i.e. electrostatic latent image support member) during each exposing process of the original document to be copied, so as to form a reference latent image for the toner concentration control thereat, and said reference latent image is developed during a developing process for forming a toner concentration control image, and further, by measuring the amount of toner adhering to said reference latent image based on light reflected from said control image by a photoelectric conversion element between a transfer process station and a cleaning process station, the amount of toner needed for the replenishment of the developing material accommodated in the developing device is controlled comparatively accurately according to said amount of adhering toner, by a known toner replenishing control circuit including a comparison circuit or the like.
The method as described above, however, still has problems in that, since the reference image is provided at the leading edge or trailing edge of the original document platform for the formation of the reference latent image, the optical system or original document platform must be subjected to additional scanning by an amount necessary for exposing the reference image, thus resulting in a larger size of the copying apparatus itself, and in that the space required for installing the photoelectric conversion element in the vicinity of the photosensitive member so as to receive light reflected from the control image, cannot be easily provided in many cases particularly in a copying apparatus employing a photoreceptor drum of a small diameter.
On the other hand, there has been developed by the present inventor, another method in which, instead of effecting an optical measurement as described above, current flowing between a developing electrode and an electrostatic latent image support member is measured during developing of the reference latent image by the magnetic brush of magnetic developing material.
The method will be described in more detail hereinbelow with reference to an arrangement of FIG. 1 which includes a rotatable photosensitive or photoreceptor drum P having a photosensitive surface Pa provided therearound, a developing sleeve S made of electrically conductive non-magnetic material and disposed adjacent to the photosensitive surface Pa of the photoreceptor drum P so as to function as a developing electrode, a magnetic roll M accommodated in said developing sleeve S, and a current measuring circuit C inserted between the developing sleeve S and the ground.
In the above arrangement of FIG. 1, during development of a reference latent image formed on the photosensitive surface Pa of the photoreceptor drum P by a developing material TC (which in this case is composed of a mixture of a high resistance magnetic carrier and an electrically insulative toner), the insulative toner adheres onto the reference latent image. During this time, current flowing between the developing sleeve S and the photoreceptor drum P is measured by the current measuring circuit C which measures the above current as a current flowing through the developing sleeve S. The current value thus measured and the toner concentration of the developing material TC are in a predetermined correlation to each other, according to which correlation, the amount of toner needed for replenishment is controlled, based on the current value as measured in the above described manner.
In the above practice, however, variation of the current value due to variation of the toner concentration is not as large as the variation in the amount of toner adhering to the reference latent image in the method earlier described, and thus, this practice is inferior to said earlier described method in the accuracy of detecting the toner concentration, with a consequent difficulty in effecting a proper control of the amount of replenishing toner. According to an analysis made by the present inventor, the above disadvantages are considered to be attributable to variation of the amount of charge on the insulative toner in a direction to cause deterioration of the detecting accuracy as shown in FIG. 2.
It should be noted here that in FIG. 2, the amount of charge on the toner has been obtained by, when the toner is charged and adapted to be electrostatically attracted onto the surface of a uniformly charged insulative sheet, measuring the amount of reduction of apparent surface potential of the insulative sheet resulting therefrom and the amount of the attracted toner, respectively. The attraction of the toner described above was effected by developing the surface of the insulative sheet.